The enzymes in lung that participate in the biosynthesis of lecithin are being studied in order to understand the biochemical basis for the presence in pulmonary tissue of large amounts of the specific lecithin, dipalmitoylphosphatidylcholine, the major surface-active component of the alveolar surfactant complex. Only in lung does this particular lecithin occur as a large fraction of the total tissue lecithins and the biosynthetic specificity for this must reside in the particular substrate requirements of key enzymes responsible for lecithin formation. Toward this end, activity requirements of rabbit lung CDP-choline: diglyceride cholinephosphotransferase (CPT) have been determined for both that in the microsomes and in the mitochondria. Various divalent metal ions are able to serve as activation cofactors, but extents of activation differ from each ion. In addition, the details of activation are different for microsomal and mitochondrial CPT activities, the latter being fully activated at lower concentrations than the other. In contrast, however, different diglycerides, varying in degree of unsaturated ion of fatty acid side chains, all seem to serve equally well as substrates for lecithin formation. Hence, the specificity that leads to large amounts of dipalmitoylphosphatidylcholine in lung does not reside in any special requirements displayed by the pulmonary CPT activity for specific diglycerides. Other possibilities for the biosynthetic specificity include several enzymes that might restructure the pulmonary lecithins after they are formed by CPT action. Determinations of substrate specificity requirements for the phospholipase A2, lysolecithin transacylase and fatty acyl CoA: lysolecithin acyltransferase activities of lung are currently underway. Also measurements of activity levels in different subcellular fractions from lung, including the microsomes, the Golgi apparatus and the lamellar bodies, are being made. Changes in levels of these enzymes also will be examined in fetal lung during gestation to see how any changes relate to the rapid increase in pulmonary alveolar surfactant that occurs just prior to birth.